Summary Autosomal dominant polycystic kidney disease (ADPKD) is the most common potentially lethal genetic disease. Eighty-five percent of the cases of ADPKD are due to mutation of the PKD1 gene of which approximately -1/3 are due to missense mutations and small, in-frame deletions that may be amenable to functional rescue. PKD1 encodes polycystin-1 (PC1), a large and complex protein thought to function as an atypical G protein-coupled receptor (GPCR). The loss of PC1-G protein regulation is now known to be fundamental to the pathogenesis of PKD, yet we know essentially nothing about the mechanism whereby PC1 regulates G protein signaling activity. Our long-term goal is to understand the mechanism underlying PC1-G protein regulation in order to target and functionally restore or augment this function as a treatment for ADPKD. Based on the structural and functional similarities between PC1 and the adhesion class of GPCRs, and intriguing results from our preliminary studies, we propose that a mechanism involving a cryptic tethered peptide ligand is responsible for the regulation of G protein signaling by PC1, and furthermore, is involved in the pathogenesis of renal cyst formation. The major goals of this proposal, therefore, are to determine the structure-function relationships behind the tethered ligand-mediated regulation of G protein signaling by PC1 and to demonstrate the relevance of this regulatory mechanism in the pathogenesis and potential treatment of ADPKD. Specifically, we intend to determine the essential properties of the tethered peptide ligand of PC1 responsible for its regulation of G protein signaling, map the regions of PC1 involved in tethered ligand- dependent signaling, and determine the conditions for the peptide ligand-dependent rescue of renal cystogenesis. The proposed studies will employ transfection of established cell lines along with standard molecular biology methods and biochemical assays to assess the signaling capability of modified expression constructs of PC1 and of soluble tethered ligand-derived peptides, and will include the treatment of cystic kidneys with soluble tethered ligand-derived peptides in both ex vivo and in vivo model systems. Successful completion of this project is expected to reveal molecular details of the regulation of G protein signaling by PC1 via its cryptic, tethered ligand, and to demonstrate the potential of this regulatory mechanism as a novel therapeutic target. Elucidating the mechanism of PC1-regulated G protein signaling will advance our understanding of the molecular pathogenesis of ADPKD, which is imperative for the development new therapies for the treatment or prevention of this disease.